The present invention relates to an offset printer or similar printer using a master.
It is a common practice with an offset printer or similar printer to use a chain-delivery type of conveying system for conveying a paper sheet or similar recording sheet. The chain-delivery type of conveying system includes a pair of parallel, toothed belts or similar chains and a clamper affixed to the chains at opposite ends thereof. The clamper clamps the leading edge of a paper sheet and conveys it in accordance with the movement of the chains.
Generally, a chain delivery type of conveying system is constructed to receive a paper sheet carrying an image thereon, i.e., a print and coveys it to a sheet discharge position, as taught in Japanese Patent Laid-Open Publication No. 11-334037 by way of example. The conveying system includes a clamper constantly biased by a spring such that the clamper tends to close. At a sheet receipt position, a cam causes the clamper to open for receiving a paper sheet and then close to clamp the paper sheet. Likewise, at a sheet discharge position, a cam causes the clamper to open for releasing the paper sheet and then close.
In a printer including a plurality of print drums, one press drum is assigned to each print drum and has substantially the same diameter as the print drum. A clamper is mounted on the press drum for clamping a paper sheet. After a paper sheet has been wrapped around the press drum, the press drum is pressed against the print drum. This type of conveying system is generally referred to as a press drum relay system.
Japanese Patent No. 2,983,661, for example, discloses a multicolor stencil printer applying the chain-delivery type of conveying system to an image transfer region. Specifically, the printer includes chains running from the upstream side to the downstream side in an image transfer positions a plurality of print drums assigned to different colors are positioned. A clamper is affixed to the chains. While the clamper clamps a paper sheet and conveys it in accordance with the movement of the chains, press rollers each press the paper sheet against associated one of the print drums. As a result, images of different colors are sequentially transferred to the paper sheet one above the other. A recess is formed in each press roller so as not interfere with a clamper mounted on the print drum or the clamper affixed to the chains. Each press roller is identical in diameter with the print drum.
A copier or similar image forming apparatus of the type using toner for forming an image on a paper sheet is also conventional. This type of image forming apparatus may include a single photoconductive element or similar image carrier and a plurality of developing devices arranged around the image carrier. The image forming apparatus is capable of printing multicolor images, including a full-color image, on paper sheets. On the other hand, an offset printer, for example, prints a multicolor image with, e.g., a plurality of print drums or similar image carriers corresponding in number to colors to be used.
An offset printer, for example, uses viscous ink and must therefore transfer the ink to a paper sheet with a high pressure. It follows that a developing device needs a number of rollers greater in diameter than rollers included in, e.g., a copier and need far higher rotation speed and torque than general office equipment. The developing device is therefore so bulky, arranging a plurality of developing devices around a single print drum has heretofore been impractical.
The present invention addresses to the following problems (1) through (6).
(1) Assume that the chain-delivery type of system is applied to an image transfer region. Then, it is difficult to define image transfer timings to a plurality of image carriers as accurately as with the press drum relay type of system. Consecutive image transfer positions are therefore apt to shift, causing resulting images to shift. Further, the chains involve a play for movement and necessarily shake during movement. The shake of the chains, however, causes a paper sheet to reach an image transfer position assigned to an image carrier while fluttering. This is apt to cause an image to blur on the paper sheet. For these reasons, the press-drum relay type of system is still predominant over the chain-delivery type of system. However, the chain-delivery type of system is advantageous over the press drum relay type of system in that it does not hand over a paper sheet and promotes highly accurate image transfer free from irregular registration.
(2) The clamper affixed to the chains is constantly biased in which it tends to close, and caused to open and close at a preselected position by a cam, as stated earlier. However, because the clamper opens and closes against a biasing force, reaction occurs during opening and closing movements. The reaction causes the chains to shake and thereby makes the receipt of a paper sheet unstable. Unstable receipt of a paper sheet directly translates into damage to the clamped portion of a paper sheet and the shift of a clamping position. When the chain-delivery type of system is applied to an image transfer region, a clamping position other than preselected one results in an image position shifted from expected one. Further, at a releasing position, the clamper is simply opened to release a paper sheet. The paper sheet is therefore apt to adhere to the clamper due to a clamping force maintained during conveyance. This is likely to prevent the clamper from releasing the paper sheet at the preselected releasing position, resulting in defective sheet discharge or a sheet jam.
(3) The press drum relay type of system is predominant over the chain-delivery type of system, as stated above. Even the chain-delivery type of system taught in Japanese Patent No. 2,983,661 and applied to an image transfer region reflects the concept of the press drum relay type of system. Specifically, a press roller having substantially the same diameter as a print drum presses a paper sheet against the print drum. The printer is therefore bulky and cannot make the most of the merits of the chain-delivery type of system.
(4) In a printer, a paper sheet is apt to adhere to a print drum due to the viscosity of ink. It is therefore likely that a paper sheet is not separated from the print drum, but rolls up in accordance with the rotation of the print drum. In the chain-delivery type of system, the clamper clamps the leading edge of a paper sheet and continuously clamps it even at a nip for image transfer, so that a separating force naturally acts on the paper sheet. However, at the position between the clamper and the nip for image transfer, the paper sheet is not fully tense, but slightly bends and slightly rolls up, as will be discussed more specifically later.
(5) Generally, in an offset printer or similar printer, an image carrier and the other arrangements must be released from each other in a preparatory step before printing, e.g., a step of depositing ink on a master wrapped around the image carrier. Although this kind of scheme is essential even with a printer of the type having a sheet conveying device arranged at one side of an image forming unit, such a construction has not been reported yet.
(6) Waterless offset printing and DI (Digital Imaging) are recent achievements in the imaging art. However, a compact design with a plurality of developing devices arranged around a single image carrier has not been realized for an offset printer or similar printer.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 9-141821, 2000-957, 2000-127333, 2000-127335, 2000-127336, 2000-141595 and 2000-44859, Japanese Patent Nos. 2,618,535 and 2,695,117 and Japanese Utility Model Publication No. 8-7402.
It is therefore an object of the present invention to provide a printer capable of performing highly accurate image transfer by solving the problems of the chain-delivery type of sheet conveying system while making the most of its merits.
It is another object of the present invention to provide a sheet conveying device capable of freeing a paper sheet from damage and the shift of an image position by obviating the shake of chains at the time of clamping, and surely releasing a paper sheet at a discharge position, and a printer including the same.
It is another object of the present invention to provide a miniature printer making the most of the merits of the chain-delivery type of sheet conveying system.
It is still another object of the present invention to provide an offset printer, stencil printer or similar printer capable of separating a paper sheet from a print drum or image carrier in a constant condition and thereby enhancing image quality.
It is yet another object of the present invention to provide a printer having a sheet conveying device arranged at one side of an image forming unit and mechanisms for releasing an image carrier and the other arrangements.
It is a further object of the present invention to provide a printer including a plurality of developing devices arranged around a single image carrier.
In accordance with the present invention, a printer includes at least one image carrier, and a sheet conveying device for conveying a sheet to which an image is to be transferred from the image carrier. The sheet conveying device includes a chain driven to run through an image transfer region, which is positioned immediately below the image carrier, from an upstream side to a downstream side in the direction of sheet conveyance, and a clamper affixed to the chain for clamping the sheet fed at a sheet feed position, which is located at the upstream side of the image transfer region.
Also, in accordance with the present invention, a sheet conveying device includes a chain driven to turn, a clamper affixed to the chain and constantly biased to remain in a closed position for clamping a sheet, and a cam device for sheet feed for causing the clamper to open, clamp the sheet, and then close. The cam device includes a cam for causing the clamper to open and then close, a guide member for preventing the chain from shaking when the clamper opens and closes.